The present invention relates to automatic pipe welding systems and more particularly to systems for welding pipe lengths end-to-end as part of a continuous pipe laying sytem.
The present invention is particularly suited for both onshore and marine pipe laying operations wherein sections of pipe must be welded end-to-end and where the pipe remains stationary and the welding mechanism rotates around the pipe ends.
The concept of welding circumferentially around stationary pipes using a rotating welding apparatus has been known in the art for some time. Examples of such prior art systems are disclosed in U.S. Pat. No. 2,960,597 issued Nov. 15, 1960 to Bruno, et al. and U.S. Pat. No. 3,084,244 issued Apr. 2, 1963 to Rieppel, et al., which are among some of the earlier patents disclosing such rotatable welding apparatus and U.S. Pat. No. 3,753,525 issued Aug. 21, 1973 to Homes and U.S. Pat. No. 3,840,170 issued Oct. 8, 1974 to Arikawa, et al., among the more recent patents dealing with this subject.
Heretofore, known pipe welding systems have been plagued by such difficulties as undue complexity, thereby increasing their operating and maintenance costs, inability to track properly, and inability to successfully weld pipe ends in accordance with American Petroleum Institute (API) standards.
The present invention is designed to overcome difficulties associated with prior art automatic welding systems. In this invention, the pipe ends are first prepared prior to welding to provide highly accurage guide surfaces. Secondly, a welding torch carriage arrangement is provided which engages the exterior surfaces of the abutting pipe ends to be welded, which carriage is guided around the circumference of the pipe ends in conjunction with the guide surfaces provided thereon. Thirdly, a back-up band arrangement is provided interiorly of the joint to be welded. These features combine in the present invention to produce a satisfactory weld joint during a single continuous pass of the welding apparatus around the pipe ends.
In its first aspect, the invention provides machined pipe ends having two sets of guide surfaces. A first radially inner guide surface cooperates with a sensor coupled to the welding torch to continuously monitor the arc gap between the welding electrode and the weld puddle. This provides a distinct advantage over previous welding operations in which the unstable weld puddle itself was used as a reference surface. In such previous systems, the appearance of a hole in the weld puddle would usually result in the torch electrode being driven into the weld joint, causing contamination of the weld and/or damage to the electrode. By using a machined surface on the prepared pipe end as a reference, the electrode to weld puddle distance can be continuously and accurately controlled as the electrode moves around the circumference of the pipe joint.
End preparation may be accomplished by clamping a pipe end with reference to the central longitudinal axis. In accord with the system described in U.S. Pat. No. 3,888,103, issued June 10, 1975 to Renshaw, the pipe end is rounded by the clamping action of multiple radial jacks exerting high forces on the pipe interior, e.g., 12 jacks exerting 30 tons each for a 36 inch pipe. Next, the cutting head of a conventional machining apparatus cuts the clamped and rounded pipe end on a plane perpendicular to the longitudinal axis within close tolerances; the cutting head will also cut the inside diameter of the annular end section (13 or 15 in FIG. 3) to a fixed inside diameter within desired tolerances.
After "end prep" of the ends of the two pipe sections for welding have been completed, the two pipe sections are clamped for welding. This may be done in the following way: one pipe end is attached to a pipe string and therefore non-rotatable; a first clamp head is clamped by multiple radial jacks to the inside of the pipe beyond the end prepped section (i.e., to the left beyond numeral 17 in FIG. 3). Simultaneously, with such clamping, the central clamp is positioned axially of the pipes so that the central clamp is midway of the pipe end. At this step, the third set of jacks is clamped into the end of the pipe section to be welded onto the pipe string, with the two prepared ends spaced about one inch apart and the pipes being concentric with each other so that the two end prepped sections (13 and 15 in FIG. 3) are concentric and "matching". Then the third clamp in the pipe section is hydraulically displaced to abut the pipe ends with suitable compressive force, e.g., about 30 tons for a 36 inch pipe. Then the jacks of the middle head are displaced radially outward from a common head an equal distance to apply an equal force at circumferentially spaced portions of the two abutting pipe ends, thereby clamping same concentric with the longitudinal axis of the abutting pipe sections. The weld-back-up ring is then expanded into circumferential engagement with the abutting prepared ends of the pipe sections, as will be described in more detail below.
This feature of "end-prepping" the pipe sections to be joined assures a true circular abutment and accurate alignment of the pipe sections for welding. This substantially eliminates bore mismatch, speeds production and improves weld quality in the pipeline.
A second aspect of the invention resides in the use of a new and improved type of welding apparatus and method. Typical tungsten inert gas via TIG or hot wire TIG, welding methods employ a tungsten electrode around which is fed a supply of inert gas to form a gas shield around the weld area to prevent oxidation of the weld nugget. In this invention, an additional flow of shielding gas may be provided around the torch electrode to augment the normal gas shield. The augmented shield provides a gas flow in opposition to pressurized air flowing through the welding chamber. This prevents exposure of the weld nugget to the pressurized air flow being maintained in the welding chamber while permitting the pressure forces to be transmitted through this shield to the weld nugget, creating a "puddle pushing" effect. The transfer of forces through the gas shield, while preventing contaminating air from entering the weld zone itself, provides a positive force for pushing the molten weld puddle through the joint to the interior surfaces of the jointed pipe ends. The use of "puddle pushing" on the root pass results in a weld bead being formed on the interior surface of the pipe; puddle pushing is not used on subsequent passes. By employing a positive force to push the weld puddle through the pipe interior in a controlled manner, a good weld can be formed around the pipe irrespective of the torch orientation; e.g., the effect of the gravitational forces on the flow of the molten weld material is reduced so as to minimize any differences between the weld bead at the top and bottom of the pipe. This allows the use of two or more (depending on pipe size) torch apparatuses to simultaneously weld circumferential sections of the pipe so as to complete a 360.degree. weld in one pass, thereby achieving a substantially uniform weld around the entire pipe.
A pressurized flow of air through the welding chamber may be used to enhance the puddle pushing effect and to facilitate movement of the welding carriage around the pipe joint. The carriage is constructed to provide flexible seal members which cooperate with the exterior surfaces of the pipes to define the welding chamber. The pressurized air in the welding chamber may flow outwardly between the seal members and pipe surfaces to create an air cushion effect between the welding carriage and the pipes. The use of this pressurized air in the welding chamber is desired but may cause difficulties if not properly used and controlled because it allows air flow in the chamber which may disturb the gas zones.
The third major aspect of the invention resides in the particular back-up band which provides a mold on the interior of the joint into which the weld puddle is flowed. This invention employs an expandable band, which may be made of any appropriate material, but preferably is copper. The back-up band is carried on a set of rounding jacks which engage the facing pipe ends and exert a circumferentially uniform radial force on the pipes. This serves to round the ends equally and bring them into coaxial alignment with each other in the vicinity of the joint. The back-up band bridges the joint and expands to form a close fitting or semi-sealed chamber around the joint. Thus, a mold is provided on the interior face of the joint, into which the weld puddle can flow.
It follows, therefore, that one object of this invention is to provide a new and improved automatic welding method and apparatus for welding stationary pipes.
A further object of the invention is to provide a system for welding from the outside the inner land of a weld joint in a single root pass.
A further object of the invention is to provide a welding apparatus that is easily mountable on the pipe ends in alignment with the joint to be welded.
A further object of the invention is to provide a means for accurately and continuously controlling the arc gap length as the welding apparatus makes a circumferential traverse of the pipe joint.
A further object of the invention is to provide a means to positively force weld material through the joint to weld the inner surface of the joint irrespective of the torch orientation.
These and further objects and advantages of the present invention will be more apparent upon reference to the following specification, appended claims and drawings.